This application is a Continuation of International Application No. PCT/JP01/03028, filed Apr. 9, 2001.
The present invention relates to a manufacturing method for shaped light metal article where an article for plastic working of light metal is plastic worked and the resulting plastic worked article is heat treated.
One method of shaping metal materials is the plastic working method called xe2x80x9cforgingxe2x80x9d. Forging is where a metal material, such as a billet, is set in a die and is hammered into a desired shape. When forging a light metal material, it is customary to subject the forged articles produced by forging to a T6 heat treatment to improve the mechanical properties. A T6 heat treatment is a two-step heat treatment composed of a solution treatment, where a high temperature is maintained for a predetermined time to increase the homogeneity of a material composition, and subsequently an ageing precipitation hardening treatment, where a comparatively low temperature is maintained for a predetermined time to increase hardness.
Cast-forging, where casting and forging are combined, is another method for shaping a light metal material. Cast-forging is where casting is performed, such as by injection molding or die casting, to produce an article for forging in a shape that is close to the intended form, with the article for forging then being forged to work the article into the intended form Japanese Laid-Open Patent Publication H11-104800 (which corresponds to European Patent Publication: EP0905266 A1) discloses a method where forged article that has been shaped using cast-forging, which is made of a light metal material, is subjected to a T6 treatment composed of a solution treatment with a processing temperature in a range of 380 to 420xc2x0 C. and a processing time in a range of 10 to 24 hours and an ageing precipitation hardening treatment with a processing temperature in a range of 170 to 230xc2x0 C. and a processing time in a range of 4 to 16 hours.
However, when injection molding or die casting is used as the casting method performed during cast-forging, internal defects, such as gas defects, are produced in the article for forging. The number of such internal defects can be reduced, such as by having semimolten metal flow into the cavity or by improving the die, but it is extremely difficult to completely eradicate such internal defects. When article for forging include internal defects, there are the problems that performing a standard T6 heat treatment after forging does not sufficiently improve the mechanical characteristics and that the appearance of the forged article is spoilt by the creation of swelling-like blisters on their surface due to the expansion of gas defects during heat treatment.
The above problems can be solved by performing a pre-forging heat treatment with the aims of converting the article for forging to a solution and expanding the gas defects, and, after the heat-treated article for forging have been forged, a post-forging heat treatment with the aim of improving the mechanical properties. With this method, the forging process ruptures and eradicates some of the blisters that appear in the surface of the article for forging due to the expansion of gas defects during the pre-forging heat treatment, resulting in a reduction in the number of gas defects present in the forged article.
However, the post-forging heat treatment is performed under the same conditions as the ageing precipitation hardening treatment that forms part of the T6 treatment. This results in the problem of the shaped light metal article produced by this method having poor ductility.
It is an object of the present invention to provide a manufacturing method which optimizes the conditions for heat treatment performed on a plastic worked article after plastic working and so produces shaped light metal article with sufficient ductility.
In order to achieve the stated object, the present invention subjects a plastic worked article made of light metal material to a post-plastic working heat treatment that has a higher temperature and shorter processing time than the ageing precipitation hardening treatment performed in a standard T6 treatment.
In more detail, the present invention is a method of manufacturing a shaped light metal article, including the steps of forming a plastic worked article by plastic working an article for plastic working made of light metal material; and subjecting the plastic worked article to a post-plastic working heat treatment at a temperature in a range of 250 to 400xc2x0 C. for between 20 minutes and 10 hours.
With the above method, a post-plastic working heat treatment that has a higher temperature and shorter processing time than the ageing precipitation hardening treatment of a T6 treatment is performed. As can be understood from the experiments described later in this specification, this enables ductility to be effectively improved, while maintaining the strength and yield strength.
A temperature range of 250 to 400xc2x0 C. is used since a sufficient improvement in ductility cannot be achieved at temperatures below 250xc2x0 C. and a significant decrease in yield strength occurs at temperatures above 400xc2x0 C.
A processing time in a range of 20 minutes to 10 hours is used since a sufficient improvement in ductility cannot be achieved by processing for less than 20 minutes and there are cases where heat treatment for more than 10 hours results in a decrease in ductility. The processing time preferably is set at 5 hours or shorter, with 1 hour being optimal.
The expression xe2x80x9clight metal materialxe2x80x9d refers to a metal, such as aluminum or magnesium, with a low density, or to an alloy of such. One specific example is AZ91D under ASTM Standards.
Plastic working here refers to forging or the like.
Even when the present kind of post-plastic working heat treatment is performed, the presence of a large number of internal defects such as gas defects in the plastic worked article subjected to this heat treatment stops the above effects from being sufficiently obtained.
In case that the light metal material is formed of light metal alloy, if the article for plastic working is subjected to a pre-plastic working heat treatment that uses a temperature that is lower than a temperature at which eutectic of the light metal alloy starts to be fused, blisters can be produced in the surface of the article for plastic working due to the expansion of gas defects included near the surface of the article for plastic working. Some of these blisters are ruptured and eradicated during the plastic working, thereby reducing the number of gas included defects in the plastic worked article. The reason that the heat treatment is performed at the temperature lower than a temperature at which eutectic of the light metal alloy starts to be fused is that at a temperature equal to or higher than the temperature, the article for plastic working is partially fused and the material composition of the fused part is not homogenized, which involves a break from the fused part at the plastic working. It is preferable for the processing temperature to be in a range of 350xc2x0 C. to 450xc2x0 C. As blisters are created before plastic working and are eradicated by the plastic working, the further creation of blisters by the post-plastic working heat treatment can be suppressed, resulting in a favorable appearance for the shaped light metal article produced by this method.
When the processing time of the pre-plastic working heat treatment is one hour or longer, blisters can be effectively produced in the surface of the article for plastic working, and, in the same manner as the solution treatment performed in a T6 treatment, the homogeneity of the material composition can also be improved. For this reason, it is preferable for the processing time to be between 10 and 20 hours.
By making both the processing time and processing temperature of the pre-plastic working heat treatment respectively longer and higher than the processing time and processing temperature of the post-plastic working heat treatment, the post-plastic working heat treatment can be performed for a short time and a low temperature, thereby suppressing the creation of blisters by the post-plastic working heat treatment.
Internal defects that are included in the article for plastic working preferably take up no more than 10% as a percentage of volume. If internal defects take up no more than 10%, a plastic worked article with extremely few defects can be obtained even when using non-fully enclosed die plastic working, which makes the complete removal of internal defects difficult. If internal defects take up more than 10%, internal defects remain after the non-fully enclosed die plastic working, so that a plastic worked article with few internal defects can only be obtained if fully enclosed die plastic working is used. This is to say, by having internal defects included in the article for plastic working take up no more than 10%, a plastic worked article with few internal defects can be obtained without placing restrictions on the method of plastic working used.
When shaping the article for plastic working, it is preferable to introduce semimolten light metal into a cavity in a die and to solidify the semimolten light metal material to shape the article for plastic working. By doing so, molten metal enters the cavity as a laminar flow or near-laminar flow. This makes it difficult for air to become trapped in the material. As a result, an article for plastic working can be produced with few internal defects, such as gas defects or shrinkage cavities. This means that high-quality article for plastic working and shaped light metal article can be manufactured. Here, the expression xe2x80x9csemimoltenxe2x80x9d refers to a state where some of the light metal material that is the raw material is still in a solid state while some of the light metal material has melted to turn into a liquid. Normally, this state can be achieved by heating a light metal raw material to below its melting point.
It is also preferable for the article for plastic working to be shaped by injection molding. This is because article for plastic working that has been shaped by injection molding has fewer internal defects due to the inclusion of air than an article produced by die casting method where atomized molten metal is used to fill a cavity in a die. Injection molding is even more effective if the molten light metal material is injected in a semimolten state below its melting point as described above.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.